Chelants or chelating agents are compounds which form coordinate covalent bonds with a metal ion to form chelates. Chelates are coordination compounds in which a central metal atom is bonded to two or more other atoms in at least one other molecule (ligand) such that at least one heterocyclic ring is formed with the metal atom as part of each ring.
Chelants are used in a variety of applications including food processing, soaps, detergents, cleaning products, personal care products, pharmaceuticals, pulp and paper processing, water treatment, metalworking and metal plating solutions, textile processing solutions, fertilizers, animal feeds, herbicides, rubber and polymer chemistry, photofinishing, and oil field chemistry. Some of these activities result in chelants entering the environment. For instance, agricultural uses or detergent uses may result in measurable quantities of the chelants being present in water. It is, therefore, desirable that chelants degrade after use.
Biodegradability, that is susceptibility to degradation by microbes, is particularly useful because the microbes are generally naturally present in environments into which the chelants may be introduced. Commonly used chelants like EDTA (ethylenediamine tetraacetic acid) are biodegradable, but at rates somewhat slower and under conditions considered by some to be less than optimum. (See, Tiedje, "Microbial Degradation of Ethylenediaminetetraacetate in Soils and Sediments," Applied Microbiology, August 1975, pp. 327-329.) It would be desirable to have a chelating agent which degrades faster than EDTA or other commonly used chelants.
Biodegradation is of particular interest in photography, but finding a commercially useful biodegradable chelant has been difficult. In the production of color photographic images, it is usually necessary to remove the silver image which is formed coincident with the dye image. This can be done by oxidizing the silver by means of a suitable oxidizing agent, commonly referred to as a bleaching agent, in the presence of halide ion, followed by dissolving the silver halide so formed in a silver halide solvent, commonly referred to as a fixing agent. Alternatively, the bleaching agent and fixing agent can be combined in a bleach-fixing solution and the silver removed in one step by use of such solution.
In the reversal processing of black-and-white photographic materials, a bleaching step is also utilized to remove photographically developed silver.
A wide variety of bleaching agents are known for use in photographic processing. For example, ferricyanide bleaching agents, persulfate bleaching agents, dichromate bleaching agents, permanganate bleaching agents, ferric chloride, and water-soluble quinones have been used. A particularly important class of bleaching agents are the aminopolycarboxylic acid bleaching agents, such as an ammonium or alkali metal salt of a ferric complex of ethylenediaminetetraacetic acid (EDTA). Ferric complex salts of propylenediaminetetraacetic acid (PDTA) having a higher bleaching power than EDTA have also been widely used as bleaching agents.
Although chelants or chelating agents, such as EDTA and PDTA, are effective in the bleaching step of photographic materials, there is interest in the photography industry to obtain chelants for use in the bleaching process which biodegrade more rapidly than EDTA and PDTA. Finding suitable chelants for use in photography, which are more biodegradable than what is commonly used, is difficult as the chelant must be able to chelate iron as well as have the proper redox ability.
Chelating ability is not indicative of redox ability of chelates of metal ions capable of more than one valence state. Nor can redox ability be predicted from structure as explained by R. Wichmann et al in "A New Bleaching Agent," presented at Imaging Science and Technology's 7th International Symposium on Photofinishing Technology, and published in R. Wichmann et al. "Advance Printing of Paper Summaries; Seventh International Symposium on Photofinishing Technology," Las Vegas, Nev., February 3-5, 1992 pp. 12-14.
Polyamino disuccinic acids have been recognized as having some chelating properties but have not received wide usage. For instance, a better known member of the family, namely ethylenediamine disuccinic acid (EDDS), has not been widely used because it has less ability to chelate certain metal ions such as calcium and magnesium than more widely used chelants. The preparation of polyamino disuccinic acids is discussed by Kezerian et al. in U.S. Pat. No. 3,158,635 where their use in rust removal is disclosed. Atkinson in U.S. Pat. No. 4,704,233 disclose use of EDDS in detergents to enhance removal of organic stains and mention its biodegradability.
EP patent application 0532003, published Mar. 17, 1993, EP application 0584665 published Mar. 2, 1994, and EP application 0567126, published Oct. 27, 1993, all disclose diamine compounds which are useful in processing silver halide light-sensitive photographic material. These compounds are reported to have improved biodegradability and safety. EP patent application 0599620, published Jun. 1, 1994, further discloses monoamine and polyamine compounds which can be used in processing silver halide-photographic light-sensitive material and are reported to have good degradation characteristics. The use of polyamino disuccinic acid chelating compounds in photographic bleach and bleach fixing solutions is further disclosed in WO 94/28464 published May 20, 1994.
It would be desirable to have a chelant, or a mixture of chelants, useful in photographic processes, particularly as a bleaching agent, when such chelant or mixture of chelants is greater than about 60 percent biodegradable within less than 28 days according to the OECD 301 B Modified Sturm Test or greater than about 80 percent biodegradable within less than 28 days according to the Semicontinuous Activated Sludge Test (ASTM D 2667 89).